Rickettsia are single-celled microorganisms that are less prevalent and produce fewer diseases than bacteria (Chapter 17). They are intermediate in size between bacteria and viruses (Chapter 18); approximately 0.3 µm in diameter and ranging from 0.3 to 0.5 µm in length. They are gram-negative, nonmotile, and nonsporing. They are easily killed by heat, dehydration, or common disinfecting agents. Rickettsia are more difficult to produce in quantity than bacteria. Similar to viruses, they

are strict obligate parasites and require living cells for growth. They cannot survive long outside a host. They also have a selective affinity for specific types of cells in the body. They are normally transmitted by an arthropod vector (i.e., ticks, lice, fleas, mites), which also serves as either the primary or intermediate host. For biological warfare purposes, rickettsia have been primarily investigated as antiper-

sonnel agents producing both lethal and incapacitating diseases. There are no known rickettsial diseases of plants. Pathogens deployed as antianimal biological weapons are generally used to produce lethal effects in an agriculturally significant species such as cows and sheep. There are no unclassified records of rickettsia being used as biological warfare simulants. Rickettsia can be stored as freeze-dried powders. In this form, they are easy to disperse.

However, because they are living organisms and can be killed during the dispersal process there are limitations to themethods that can be used. They can also be stored and dispersed via infected vectors (e.g., lice, ticks). In most cases, large-scale attacks will be clandestine and only detected through epidemiological analysis of resulting disease patterns. Localized or small-scale attacks may take the form of “anthrax” letters. Even in these cases, without the inclusion of a threat the attack may go unnoticed until the disease appears in exposed individuals (e.g., the initial 2001 anthrax attack at American Media Inc., which claimed the life of Robert Stevens). It is possible that local insects can become both a reservoir and a vector for the pathogen.

Under these circumstances, the pathogen can survive well after the initial release and can rapidly spread beyond the immediately affected area. In many cases, once a vector is infected, it is capable of transmitting the disease throughout its life span. Some pathogens are transmitteddirectly to theyoungof thevector so that thenextgeneration isborn infected. Response activities must also include efforts to contain and eliminate these vectors (e.g., application of pesticides).